Ceramide, which is present in the horny layer of the skin, forms a lipid barrier necessary for retaining moisture and plays an important role in the maintaining of moisture in the skin.
Ceramide in the horny layer is produced by the breakdown of cerebroside by an enzyme known as cerebrosidase. The ceramide is partially transformed into phytosphingosine and sphingosine by an enzyme known as ceramidase. Phytosphingosine and sphingosine play an important role in the control of cell growth and differentiation. Six different types of ceramides possessing different functions are present in the human skin.
However, since ceramides are highly crystalline, have a low solubility in other oil components, and produce crystals at a low temperature, it is difficult to ensure long-term stability of cosmetics comprising a ceramide.
Oil-in-water (O/W) or Water-in-oil (W/O) emulsions are well known in the field of cosmetics and dermatology, in particular for the preparation of cosmetic products, such as milks, creams, tonics, serums or toilet waters.
In particular, a fine emulsion such as an O/W nano- or micro-emulsion is particularly interesting in cosmetic products due to its transparent or, slightly translucent aspect.
For example, JP-A-H09-110635 discloses a fine emulsion which is formed by using a combination of polyglyceryl fatty acid ester, as a surfactant, and C10-C22 2-hydroxy fatty acid. In addition, JP-A-H11-71256 discloses a fine emulsion which is formed by using a combination of polyglyceryl fatty acid ester and a betain.
However, in order to prepare a fine emulsion including ceramide, in general, a large amount of a surfactant is required, which impairs safety and feeling during use.
If the amount of surfactant is decreased in order to obtain an excellent feeling during use, the ceramide does not solubilize transparently, resulting in a cloudy or opaque emulsion in many cases. In these cases, separation and creaming occur over time and it is difficult to obtain adequate long-term stability, in particular at an elevated temperature.